Device-to-device communication (D2D) is a component of existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems typically operate in unlicensed spectrum. D2D communications may also operate as an underlay to cellular networks as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Such device-to-device communication may share the same spectrum as the cellular system by, for example, reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes, however, is not a desirable solution as spectrum is a scarce resource. Dynamic spectrum sharing between the device-to-device services and cellular services provides flexibility and higher spectrum efficiency.
D2D communication may offer a variety of services to peer devices. Some non-limiting examples of such services include public safety and disaster relief (also known as national security and public safety (NSPS)), relaying function for coverage extension, proximity based social networking, cooperative positioning, and so on. D2D applications may include direct discovery and direct communication. In both cases, a transmitter sends D2D signals that are directly received at least by the intended receivers. D2D devices may operate in multi-carrier scenarios where cellular and/or D2D is configured to operate on multiple carriers. Such carriers do not necessarily belong to a single operator and are not necessarily coordinated and synchronized. D2D devices may operate on a carrier that is not the carrier of the device's serving cell.
D2D devices may use a D2D discovery process in which the device may advertise its capabilities and/or search for other devices capable of D2D communication. Signal quality measurement may be used for D2D communication and discovery. For example, signal quality measurements may be used to select synchronization signals.
D2D communication may refer to direct communication between devices. D2D devices operate within a radio access network. In cellular network assisted device-to-device communications (also called D2D communications as a cellular underlay), user equipment (UE) in the vicinity of each other can establish a direct radio link (D2D bearer). While UEs communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station (eNB).
In this way the cellular radio access network (RAN) can assist and supervise the UEs in allocating time, frequency, and code resources for the D2D bearer. Also, the cellular network may determine whether the D2D pair should use the direct link or communication should take place via the eNB. The network may also set the maximum power level that the D2D pair may use for the D2D bearer. Thus, network assisted D2D communications may take advantage of the short distances between devices and reuse cellular spectrum while at the same time protecting the cellular layer from potentially harmful interference caused by the D2D link.
Before establishing D2D communications, a D2D device may detect nearby devices capable of D2D communication. D2D devices may transmit discovery signals that may be detected by other D2D devices.
D2D communication may operate in multicarrier or carrier aggregation (CA) networks. In multicarrier or carrier aggregation networks, the UE is able to receive and/or transmit data to more than one serving cell (i.e., a CA capable UE may operate with more than one serving cell).
The carrier of each serving cell may be referred to as a component carrier (CC). A component carrier generally refers to an individual carrier in a multi-carrier system. Carrier aggregation may also be referred to as “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, or “multi-carrier” transmission and/or reception.
Carrier aggregation may transmit signaling and data in both the uplink (UL) and downlink (DL) directions. One of the component carriers is the primary component carrier (PCC) (also referred to as primary carrier or anchor carrier). The remaining carriers are called secondary component carriers (SCC) (also referred to as secondary carriers or supplementary carriers). The serving cell may be interchangeably referred to as primary cell (PCell) or primary serving cell (PSC). Similarly, the secondary serving cell may be interchangeably referred to the as secondary cell (SCell) or secondary serving cell (SSC).
Generally, the primary or anchor component carrier carries the essential UE specific signaling. In carrier aggregation, the primary component carrier (e.g., PCC or PCell) exists in both uplink and downlink directions. In a scenario with a single uplink component carrier, the single uplink is the PCell. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The serving radio network node (e.g., eNodeB in LTE) may use a configuration procedure to configure a carrier aggregation UE with one or more SCells (DL SCell, UL SCell, or both). The eNodeB may use a de-configuration procedure to de-configure or remove one or more already configured SCells (DL SCell, UL SCell, or both). The configuration or de-configuration procedure may also be used to change the current multi-carrier configuration (e.g., for increasing or decreasing the number of SCells or for swapping the existing SCells with new ones).
D2D UEs transmit D2D signals or channels in the uplink part of the spectrum. A D2D UE may operate in a half-duplex mode (i.e., the UE can either transmit D2D signals/channels or receive D2D signals/channels). D2D relay UEs may relay some signals to other D2D UEs. D2D signals may include control information, some of which is transmitted by D2D UEs and some of which is transmitted by eNodeBs (e.g., D2D resource grants for D2D communication may be transmitted via cellular downlink control channels). D2D transmissions may occur on resources which are configured by the network or selected autonomously by the D2D UE.
D2D communication refers to transmitting, by a D2D transmitter, D2D data and D2D control information with scheduling assignments (SAs) to assist D2D receivers of the D2D data. D2D data may be transmitted according to configured patterns and may be transmitted relatively frequently. Scheduling assignments may be transmitted periodically. In some examples of operation, D2D transmitters that are within the network coverage may request eNodeB resources for their D2D communication transmissions and receive in response D2D resource grants for scheduling assignments and D2D data. In other examples of operation, an eNodeB may broadcast D2D resource pools for D2D communication.
D2D discovery messages may be transmitted in relatively infrequent periodic subframes. An eNodeB may broadcast D2D resource pools for D2D discovery, both for reception and transmission.
Power control is a consideration for D2D communication. In LTE, uplink power control is specified. To enable uplink power control operation, a UE may be configured with one or more parameters. For example, the UE may derive path loss (PL) based on cell reference signal (CRS) power and reference signal receive power (RSRP) measurement on a serving cell in which it operates.
A carrier aggregation UE may independently perform power control in each serving cell (i.e., PCell and SCell(s)) for different control channels. Path loss may be derived based on RSRP of the downlink serving cell (i.e., DL CC), which is linked to an uplink serving cell (i.e., UL CC) for which the power control is performed. The linkage between the downlink and uplink carriers may be signaled to the UE by a higher layer.
D2D Synchronization Signals (D2DSS) may be used for D2D communication and discovery. A D2D wireless device may transmit D2DSS in D2DSS resources configured by an eNB. The D2D resources may include a periodically occurring subframe in which a D2D wireless device may transmit D2DSS if certain conditions are satisfied.
As a particular example, an in-coverage UE may transmit D2DSS in a D2DSS resource if either of the following conditions are satisfied: (a) if the subframe is within the SA or D2D data period in which SA or data is transmitted, the UE is RRC_Connected, and the eNB has instructed the UE (e.g., by dedicated signaling) to start D2DSS transmission; or (b) if a RSRP threshold for communicating D2DSS transmission is configured, the RSRP value of the UE is less than the threshold, and the eNB has not instructed the UE (e.g., by dedicated signaling) to stop D2DSS transmission. A D2DSS may also be referred to as a sidelink synchronization signal (SLSS). An SA may also be referred to as Physical Sidelink Control Channel (PSCCH).
A D2D UE may perform D2DSS signal quality measurement for synchronization determination, link adaption, power control and/or other Radio Resource Management (RRM) or Radio Link Monitoring (RLM) functionalities. As described above, however, D2DSS may only be transmitted by a particular group of UEs which meet particular conditions. Thus, reliance on D2DSS measurements may be difficult when the D2DSS may not always be present. For example, a D2D UE may not be able to obtain accurate measurements if sufficient D2DSS measurement samples cannot be obtained.